Water heater problem

On Atwood water heaters anyway, the DSI light will light up if there's a thermostat failure and the ECO opens on both LP and electric. One thermostat set, one error light.

I am thinking you have a straight DSI water heater. Dometic appears to have purchased Atwood by a little search.
I would check the eco and thermostat where they are mounted to tank. Make sure they are tight against tank.

Check the burner area for debris. Make sure the ignitor/flame sensor are adjusted correctly.
Yes, please post model number!

So, being as you are so new here. You likely have a six gallon hot water tank. That really is quite a lot of hot water. Assuming you have the tank full, and can then check what happens.

Do a measurement of a gallon in your kitchen sink, then see what the sink will hold. Assuming it might hold three gallons.... run pure hot water into the sink. As the hot water is expelled out of the top of the hot water tank, cold water enters into the bottom and mixes somewhat.

Do you get three gallons of hot hot water out of the tank? You should. If not, what happens. I am wondering whether you have a valve in the wrong position or a check valve stuck somehow.

It doesn't really explain the fault code, but it might be a place to start.

Not really sure why the DSI light would come on if you're running it on electric, are you running it on electric AND propane?

It would help to know the make and model of the heater. It could be a thermostat closed failure and the over temp emergency cut off (ECO) is tripping to cut off the heat.